Due to their high energy-to-weight ratio, safety of use, and other advantages, metal-air, and particularly zinc-air, batteries have been proposed as a preferred energy source for use in electrically powered vehicles.
To date, much of the development concerning use of metal-air batteries as a main power source for vehicle propulsion has focused on modified "mechanically rechargeable" primary battery systems comprising a consumable metal anode and a nonconsumable air cathode. The metal anode is configured to be replaceable once the metal component therein is expended following oxidation in the current producing reaction. These systems obviously constituted an advance over the previously proposed secondary battery systems which have to be electrically charged for an extended period of time once exhausted, and require an external source of direct current.
Some of these mechanically rechargeable systems, such as the one disclosed in U.S. Pat. No. 4,139,679 to Appelby are quite complex in construction, incorporating an active particulate metal anode component freely suspended in an alkaline electrolyte, and a pump to keep the particulate metal anode in suspension and circulated between air cathodes. After discharge of the metal anode component, the electrolyte is then replaced with an electrolyte containing a fresh particulate metal anode component in suspension.
Other prior art battery systems, such as the one disclosed in U.S. Pat. No. 3,436,270 to Oswin, comprise electrochemical cells utilizing a fixed planar anode configured for easy replacement and placed in close adjacency to one or more air cathodes, physically separated therefrom by fluid permeable protective screens, but kept in current producing contact with the cathode by an alkaline electrolyte. Referring to FIG 1., in such prior art devices, a metal anode element denoted 10 generally comprises a central corrosion resistant current collector planar metallic mesh or foil frame 12 attached to a base member 14, and a terminal 16. An anode 18 consisting of a laminated sheet metal or porous metal plate or a viscous slurry of active metallic particles, typically zinc, impregnated with electrolyte is spread over frame 12. Once the metal anode element 10 is entirely discharged, it is removed from the cell and replaced by a fresh anode element. These systems have been particularly heralded for use in electric vehicle propulsion since they facilitate quick recharging of the vehicle batteries simply by replacement of the spent anodes, while keeping the air cathodes and other battery structures in place. This mechanical recharging, or refueling may be accomplished for instance in service stations dedicated to the purpose. To further enhance the cost efficiency of such systems, it has been proposed in our above copending application to regenerate the metal anode at an external plant by chemical recycling process so that it may be reformed into a fresh anode element for later reuse in either the same or a different cell.
Despite the obvious advantages of such a primary mechanically rechargeable system, it appears desirable to offer the electric vehicle owner a further option of occasionally electrically recharging a normally mechanically rechargeable primary battery, since electrical recharging may offer: (i) a potentially lower marginal cost per recharging, (ii) where the owner has sufficiently available time, increased convenience since the need to go to a battery refueling/service station is reduced, and (iii) reduced overall impact on battery refueling/service station infrastructure requirements. Such electrical recharging would be as applicable with respect to secondary batteries, that is by reversing the direction of current flow by applying direct current to the cells.
However, converting mechanically rechargeable primary cells into electrically rechargeable secondary cells is considerably more difficult than merely reversing the direction of current flow by applying current to the cell to recharge the spent electrodes.
As known in the art, one of the principal advantages of the metal-air secondary battery lies in the fact that only the metal anode requires recharge. The air cathode, since it relies on electrochemical reduction of ambient oxygen by a static catalyst for the comsumption of electrons in the current producing reaction, need not be regenerated.
Studies have shown that attempts to adapt prior art metal-air cells comprising a structured metal anode for electric regeneration as secondary cells have met with three principal problems: (1) uneven replating of the metal anode following the electric recharge process, particularly slumping of the metal anode element to the bottom portions of the anode as illustrated in FIG. 2; (2) formation of dendrites generally perpendicular to the recharging cathode which eventually short out the cell by bridging between the metal anode and air electrode; and (3) degradation of the air electrode arising from production of oxygen in the recharging process causing oxidation and corrosion of the electrode, and delamination of its components, as well as a build up of internal pressure within the cell tending to force electrolyte into contact with the outside environment resulting in contamination and evaporation.
As a result of a combination of the above problems, proposed fixed anode metal-air batteries suffer from a reduction of efficiency following electrical recharging, requiring additional complex and expensive adaptive means to retard the negative effects of recharging. In the absence of such adaptations, the entire cell or battery must be replaced following only a very limited number of recharge cycles. Moreover, even the longest life rechargeable metal-air batteries are subject to severe degradation of the metal anode following repeated electrical discharge/recharge cycles. Such degradation eventually leads to irreversible damage of the anode thus requiring that the entire cell or battery be replaced. Replacement may be required even though other battery structures including the housing, cathodes and current collectors may still be perfectly usable, which of course is wasteful.
In our copending U.S. application No. 07/636,411 filed on Dec. 31, 1990 it has been proposed to electrochemically regenerate spent metal anode active material external to the battery. However, even when severe but less than irreversible degradation is caused by the above described processes of repeated inplace electrical discharge/recharge, it has been found that the external regeneration is made more lengthy, difficult and costly. In extreme instances, external regeneration may be even rendered impossible.
Prior art, U.S. Pat. No. 3,650,837 and U.S. Pat. No. 3,759,748 both to Palmer, teaches attempts to solve the problem of air electrode degradation by providing improved air electrodes incorporating improved catalysts and composite construction to reduce destruction upon recharging. U.S. Pat. No. 4,957,826 to Cheiky, teaches means to prevent electrolyte leakage and contamination. U.S. Pat. No. 4,842,963 to Ross offers a particular solution to the problem of dendrite formation in a fixed non-replaceable anode of an electrically rechargeable metal-air cell. However, the prior art fails to teach a comprehensive solution to the above problems, particularly preventing the complete and irreversible exhaustion of the metal anode or otherwise providing for adaptation of primary mechanically rechargeable metal-air batteries to be repeatedly electrically recharged.